This invention relates to radio communication of data such as in a remote tire monitoring system of a vehicle.
U.S. Pat. No. 5,600,301, assigned to the assignee of the present invention, discloses a tire pressure monitoring system that includes sending units associated with each tire of a vehicle and a central receiving unit. Each of the sending units senses tire pressure and transmits a radio frequency (RF) signal. The RF signal includes an identifier and an indication of tire pressure. The receiving unit receives the RF signal and decodes the identifier and the tire pressure indicator. Abnormal tire pressure conditions may be identified for the user in this manner.
The tire monitors in such a system are conventionally mounted on the wheels of the vehicle within the tire. The tire monitor is enclosed within a housing within the tire. A valve stem is integrally associated with the tire monitor and extends through a hole in the wheel. In some cases, the valve stem is used to form an antenna for transmission of the RF signal.
This system has worked well in use with tires having sidewalls made of synthetic materials. Tire manufacturers, however, are developing tires having metal such as steel in the walls of the tires. Such tires are referred to as runflat because they provide the capability to continue travelling even when deflated. This is done by increasing the strength of the sidewall by steel reinforcement.
It has been discovered that conventional tire monitoring systems do not work well in use with such metallic ply construction tires. The transmitted signal is severely attenuated and is not received reliably or at all by the system receiver. This is a particular concern with runflat tires because the user may not be aware of a reduced pressure or flat tire condition without the tire pressure monitoring system's warning.
U.S. Pat. No. 4,953,393 discloses a tire monitor including a passive monitoring means having a characteristic resistance, inductance and capacitance, creating a natural frequency of oscillation. Variations in tire pressure will change the inductance or capacitance, thereby changing the natural frequency of oscillation. The monitoring means is electrically excited and the variable oscillation frequency is used to determine tire pressure. However, no accommodation is made for the characteristic frequency response of the tire due to metallic components.
U.S. Pat. No. 5,569,141 discloses a tire pressure monitoring system in which a tire vibration frequency components are derived from signals indicating wheel speed. Using resonance frequencies, tire pressure can be detected. However, again no accommodation is made for the frequency response of the tire when it includes metallic components which limit or attenuate radio transmission.
Accordingly, there is a need for a method and apparatus insuring reliable transmission and reception of data such as tire data for use with metallic ply tires.